The present invention relates to a filter circuit. More particularly, the present invention is suitably applied to a high-pass filter circuit that allows only a signal of a high band frequency to pass therethrough, formed in an IC chip.
Well-known filters are categorized into low-pass filters (LPF), high-pass filters (HPF), etc., according to frequency selection performance. A low-pass filter is a filter which allows signals of frequencies below a selected frequency to pass therethrough while blocking signals of frequencies higher than the selected frequency. On the other hand, a high-pass filter is a filter which allows signals of frequencies higher than a selected frequency to pass therethrough while blocking signals of frequencies lower than the selected frequency.
FIG. 1 shows the configuration of a conventional high-pass filter based on the simplest CR circuit. As shown in FIG. 1, the conventional high-pass filter is constituted by a capacitor 101 connected between a signal input end IN and a signal output end OUT, and a resistor 102 connected to the output terminal of the capacitor 101 in parallel with the output end OUT.
The cutoff frequency fc of the thus-arranged high-pass filter, which is the point of division between the pass band and the attenuation band, is obtained from the capacitance value C of the capacitor 101 and the resistance value R of the resistor 102 as shown by the following equation (1):
fc=1/(2xcfx80CR)xe2x80x83xe2x80x83(1)
In the case of reducing the cutoff frequency fc of the above-described conventional high-pass filter (increasing the pass band), it is necessary to increase the capacitance value C or the resistance value R. A capacitor having a large capacitance value C or a resistor having a large resistance value R (a resistance of several hundred kilohms or higher in particular), however, requires a considerably large area when incorporated in an integrated circuit (IC). Thus, with ICs incorporating a high-pass filter, there has been a program that the entire circuit area of the IC is increased.
That is, the upper limit of the capacitance value C of the capacitor 101 realizable in an integrated circuit is about several hundred picofarads and the upper limit of the resistance value R of the resistor 102 is about several hundred kilohms. It is difficult to incorporate the capacitor 101 or the resistor 102 in an integrated circuit if this value is exceeded. For this reason, the capacitor 101 having a larger capacitance value C or the resistor 102 having a larger resistance value R constituting a high-pass filter of a lower cutoff frequency fc has been realized as an external circuit for the IC.
Ordinarily, manufacturing variation of a resistor is considerably large and a variation of about xc2x130% to xc2x1100% exists in a desired resistance value R. There has been a problem that a variation is also caused in cutoff frequency fc of a high-pass filter formed by using such a resistor and the stability of the transfer frequency characteristics of the filter is thereby reduced. There has also been a problem that the stability of a temperature characteristic of the high-pass filter is also reduced by such a manufacturing variation of the resistance value R.
The present invention has been achieved to solve such problems and an object of the present invention is to provide a high-pass filter having a small circuit area even in the case of reducing the cutoff frequency, having smaller manufacturing variation, and suitable for incorporation in an IC.
A filter circuit of the present invention is characterized by comprising a capacitor connected between a signal input end and a signal output end, a first constant current circuit of a MOS structure connected between the signal output end and a power source, and a second constant current circuit of a MOS structure connected between the first constant current circuit and the ground, and characterized in that the first constant current circuit has first and second pMOS transistors connected in a current mirror structure to the power source, the drain of the first pMOS transistor being connected to the signal output end.
In still another mode of implementation of the present invention, the second constant current circuit comprises a first nMOS transistor having its drain connected to the drain of the second pMOS transistor, and its gate connected to the drain of a second nMOS transistor, a first resistor connected between the source of the first nMOS transistor and the ground, the second nMOS transistor having its source and drain respectively connected to the ground and a second resistor, and the second resistor connected between the drain of the second nMOS transistor and the ground.
In still another mode of implementation of the present invention, a third nMOS transistor is provided which has its gate connected to the output node of the capacitor, and its source and drain respectively connected to the ground and the signal output end.
The present invention comprises the above-described technical means and is therefore capable of forming a high-pass filter by using a constant current circuit of a MOS structure, and capable of reducing the cutoff frequency of the filter by increasing the equivalent resistance value of the constant current circuit constituted by a MOS circuit without using a capacitor of a large capacitance value or a resistor of a large resistance value. The equivalent resistance value of the constant current circuit is increased by adjusting the value of a current caused to flow through the constant current circuit.
Thus, it is possible to provide a filter circuit having a low cutoff frequency and suitable for incorporation in an IC without increasing the circuit area. The equivalent resistance value of the constant current circuit can be changed, for example, by adjusting the gate areas of the MOS transistors constituting the MOS circuit. Since the value of the cutoff frequency is determined according to the gate areas of the MOS transistors, etc., manufacturing variation can be limited to a small value and the filter can be stabilized in temperature characteristic.